1. Field of the Invention
The invention concerns a tunable power laser and, notably, a power laser source emitting at a wavelength that represents no danger to the human eye.
The field covered by the present invention concerns the making of power laser sources working in a band ranging from 1.54 .mu.m to 10 .mu.m.
The choice of the emission wavelength in a spectral region located in a band higher than 1.5 .mu.m is related to the fact that the risk of optical damage to the eye is reduced to the minimum and that, at this wavelength, the atmosphere offers a good window of transmission. It may be recalled that the maximum exposure for the human eye is 5 .mu.J/cm.sup.2 at 1.064 .mu.m, and goes to 1 J/cm.sup.2 at 1.54 .mu.m. It follows from this that the making of power laser sources in this spectral region has undeniable advantages.
2. Description of the Prior Art
However, there are no materials displaying a matrix effect nor any rare earths that permit usable laser transmissions, apart from the ion Erbium.
The drawback related to the use of this rare earth is that the laser operation is described by a system with three levels, and this entails a number of disadvantages (high threshold, superimposition of the laser transition on an absorption band, easier saturation of the gain etc.).
In another method, this kind of source is made by Raman transfer. In this case, an Nd:YAG (neodymium-doped YAG) laser is used. This Nd:YAG laser pumps a cell in which there is a gas with a Raman frequency shift permitting the pump wave, which is at 1.064 .mu.m, to be transferred towards 1.54 .mu.m. Such a gas may be methane (CH.sub.4) under high pressure.
This gas has a Raman spectral shift of 2916 cm.sup.-1 and a Raman gain coefficient d of the order of 1.4 cm/GW under a pressure of 10 atmospheres.
A description of such sources, using the Raman effect, will be found in the following articles:
D.C. HANNA, "A High Power Short Pulse Stimulated Raman Source at 1.54 .mu.m" in Optics Communication, vol. 60, No. 3, Nov. 1, 1960; PA1 D.C. HANNA et al, "Stimulated Raman Scattering of Picosecond Light Pulses in Hydrogen, Deuterium and Methane" in IEEE Journal of Quantum Electronics, vol. QE-22, No. 2, February 1986; PA1 J.J. OTTUSCH et al, "Measurement of Raman Gain Coefficients of Hydrogen, Deuterium and Methane" in IEEE Journal of Quantum Electronics, vol. 24, No. 10, October, 1988. PA1 a pump laser source emitting an optic pump wave (F.sub.p) at a first determined wavelength; PA1 a pressurized gas cell containing a gas of a nature such that, receiving the pump wave (F.sub.p), the cell emits a Stokes wave (F.sub.R) at a second wavelength which is different from the first wavelength; PA1 a non-linear crystal receiving the Stokes wave (F.sub.R) at a determined angle with respect to the optical axis of the crystal and giving, in exchange, two output waves (F.sub.s, F.sub.i) with wavelengths that are different from the second wavelength.
Thus, with a pumping centered on the wavelength of emission of the Nd:YAG laszer, it is possible to generate an emission at the Stokes wavelength: ##EQU1## obtained from the relationship: EQU W.sub.R =W.sub.p -.DELTA.W.sub.R
W.sub.p, .DELTA.W.sub.R, W.sub.R being respectively the pump angular frequency and the frequency of the Stokes wave. In terms of wavelength and for a given Raman shift in cm.sup.-1, we obtain: EQU .lambda..sub.R =(1/.mu..sub.p (cm)-2916 cm .sup.-1).sup.-1
However, a source such as this cannot be used to tune the wavelength emitted to a determined wavelength.
The invention therefore puts a source such as this into application and provides means enabling a source such as this to be made tunable.